1. FIELD OF THE INVENTION
This invention relates to an electron producing apparatus and more particularly to an electron producing apparatus of the field emission type.
2. DESCRIPTION OF THE PRIOR ART
In an electron producing apparatus using field emission phenomenon, the current due to field emission, when produced through the application of a high voltage to the field emission electron source with its electron emission surface cleaned, for example, by heating, first varies in an initial unstable region wherein the field emittion current steeply decreases and then reaches a first stable region wherein the current first assumes a substantial constant value without any appreciable decrease and then gradually increases. After the stable region, the emitted electron current enters a second unstable region wherein it increases while fluctuating. Thereafter, the current further increases with greater fluctuations and the further continuation of field emission will damage the field emission source by severe discharge. For this reason, it is necessary to interrupt the field emission as soon as the later unstable region has been reached and to re-clean the field emission source by heating for example. The current due to field emission, when resumed after the cleaning, will undergo almost the same change with time. Conventionally, it was the operator who manually interrupted the field emission and cleaned the emission source through the monitor of the emitted current. In this case, however, there was a drawback, i.e. a possibility that the field emission source might be injured due to the operator's carelessness.
An electron producing apparatus of the field emission type, provided with a control means for eliminating such a drawback as mentioned above is disclosed in Japanese Pat. application No. 93,993/45, entitled "FIELD EMISSION APPARATUS" and filed Oct. 27, 1970, the subject matter of which was invented by T. NAGATANI and then the right to obtain a patent thereto was assigned to the common assignee to the present application. The Japanese Pat. application No. 93,993/45 has been registered as Japanese Pat. No. 719,194 under the date of Feb. 19, 1974. The electron producing apparatus of the field emission type disclosed in the specification of the Japanese Pat. No. 719,194 is provided with a control means by which instantaneous values of the electron current emitted from a field emission electron source or an electron gun are sequentially measured and the field emission source or the electron gun is deenergized so as to interrupt the emission of electrons when an instantaneous value of the current at a certain point of time assumes a predetermined magnitude relative to an instantaneous value previously measured at the moment when a predetermined time has lapsed after the initiation of the electron emission by the electron gun. However, the mode of change with time in the current due to the field of emission of electrons is not unique but varies largely depending on the degree of vacuum around the electron gun, the sort or amount of the residual gas and the magnitude of the emitted electron current. The time period from the initiation of the field emission to the end of the later or second unstable region, ranges from a few minutes to several tens of hours. The control means therefore has a drawback, i.e. practical inconvenience, that the above-mentioned predetermined time must be set anew each time the degree of vacuum, the quantity or sort of the residual gas, or the magnitude of the current due to the electron emission is changed for some reason.